The present innovation is generally directed toward Monolithic Microwave Integrated Circuit (MMIC) power amplifiers. More specifically, the present innovation is directed to a method for creating a single-chip MMIC or RFIC using the principles of a Doherty power amplifier.
The principle of Doherty amplifiers was shown as early as 1936. However, practical designs utilizing a single RFIC have not been feasible in earlier implementations. This patent application is directed toward unique features to allow the possibility of a single-chip RFIC using the principal of a Doherty power amplifier.
Power amplifiers are typically less efficient as the output power is backed off from the maximum achievable output power.
The present invention is generally directed to the design and implementation of complex circuits to reduce distortion and linearize the output corresponding to radio frequency (RF) and microwave frequency ranges. An embodiment of the present integrated circuitry (IC) is configured to use a combination of bipolar transistors, FETs and resonant circuits to effectively provide lineariztion and dynamic power control for RF and microwave signals. Feasible combinations of bipolar transistors and FETs include GaAs Heterojunction Bipolar Transistors (HBTs) with PHEMTs, as well as Silicon Germanium Bipolars with CMOS.
Today, one problem with operating a power amplifier at low distortion levels is that the efficiency of the amplifier circuit is greatly reduced because it is not amplifying at its highest capability. Therefore, a circuit according to the present system can preferably utilize a bias circuit to adjust the bias characteristics and subsequent power amplification corresponding to changing circuit conditions.
Specifically, in one embodiment, a bias circuit may preferably utilize a leakage current of an RF input to determine the state of amplification and accordingly adjust the bias. By adjusting the bias conditions and power amplification characteristics to the existing conditions, power amplification efficiency is greatly improved.
Demands on the linearity and efficiency of power amplifiers are common in radio frequency and microwave communication systems. Conventionally, power amplifiers normally operate at maximum efficiency at or near saturation. However, in order to accommodate the linearity of today's devices with communication signals having varying amplitudes (e.g. cellular telephones), systems utilizing conventional power amplifiers normally operate at less than peak efficiency for a substantial portion of the time.
Of course, in today's market, wireless communications devices, such as cellular telephones and wireless local area networks (WLAN), must consistently provide clear and undistorted transmissions. As well, the batteries in the devices must be small in physical size while maintaining a long operating life.
In order to meet these consumer requirements, wireless designers and engineers have moved away from using traditional silicon-based bipolar transistors in power amplifiers and toward using more exotic transistors, such as heterojunction bipolar transistors (“HBTs”) made of aluminum-gallium-arsenide/gallium-arsenide (“AlGaAs/GaAs”) and indium-gallium-phosphide/gallium-arsenide (“InGaP/GaAs”). Such HBTs provide outstanding power efficiency and high linearity, thus enabling wireless devices to achieve longer battery life and better signal characteristics for voice and data.
Additionally, the trend in data networks is to provide higher data rates with complex modulation schemes. Complex modulation schemes require the design and implementation of linear systems in order for data transmission to be successful. In most cases, linear system design places significant constraints on individual circuits within the system.
Quite often the modulated signals applied to the system have very high peak-to-average-power-ratio (PAPR) which requires the individual circuits in the system to be designed so that they can withstand a large range of power levels. Essentially, the individual circuits must be designed with a large dynamic range, which makes the circuits inefficient and expensive.
Ultimately, circuit linearization techniques provide solutions to problems associated with signals requiring large dynamic ranges. Earlier techniques that implement feedback, predistortion, feedforward, and other signal processing concepts have not been feasible for use in RFIC designs. The technique used in the one embodiment of the present innovation allows for linearization and dynamic power control to be implemented in RFICs by manipulating aspects of the bias circuitry and through a unique application of the mixture of device technologies now available on the same integrated circuit.
Thus, what is needed is an approach that achieves higher efficiency as a function of the power backoff than normal classes of power amplifiers.